Q--I have a 2006 Chrysler 300C with the HEMI engine that uses the cylinder deactivation system to save gas on the highway. Recently I drove to Denver from Dallas and back. I noticed that from Dallas (elevation 600 feet) to Amarillo (elevation 3600 feet) I got about 19.5 mpg with no headwind and a gentle slope up. From Amarillo to Raton, New Mexico (elevation 6700 feet) I got 21.3 mpg, and finally from Raton to Denver (elevation 5200 feet) I got 23.6 mpg. This was all driving about 80 mph. I know the computer leans out the mixture at high altitude to compensate, and there was also a significant power drop at 6000 feet vs. 600, and it would seem that the car should get better mileage when the engine is most efficient. I am not complaining about the mileage with 340 horses and a 4200-pound car, but I am curious about a more than 20-percent improvement in mileage at higher altitude.

A--Fuel economy improves at higher altitude in all engines because the pumping work is reduced. As you go up in altitude, a lower manifold vacuum is required to move the car. The lower vacuum means lower pumping work and the result is better fuel economy. This also explains the reduced power. Think of an engine as an air pump and you may start to get the picture.

Since you were driving at about 80 mph most of the time, the cylinder deactivation would not kick in, so that is not a factor.

Regarding leaning out the fuel mixture, you are probably old enough to remember when carburetors needed to have smaller jets installed at higher altitudes to keep the engine running right. They were not leaning out the fuel mixture, but lowering the amount of gas delivered to more closely match the amount of air. Today, the computer maintains the air/fuel mixture at precisely 14.7 pounds of air to 1 pound of fuel (stoichiometry) no matter the altitude. It’s not leaner; it’s just right.